/Engine/Source/Editor/UMGEditor/Public/WidgetBlueprintEditorUtils.h
static FVector2D GetWidgetPreviewUnScaledCustomSize ( FVector2D DesiredSize, [UUserWidget](API\Runtime\UMG\Blueprint\UUserWidget) * UserWidget, [TOptional](API\Runtime\Core\IO\TOptional)< FVector2D > ThumbnailCustomSize, EThumbnailPreviewSizeMode ThumbnailSizeMode )